The above-described package is normally formed of a flat blank by pressing it to shape in a mould. Thus, the coating of the cardboard, which forms the inner surface of the package, also forms the upper surface of said flange which will come into contact with the lid.
Sometimes there is a need to provide the edge flange with a material that is suitable for fixing the lid but is not necessarily required in the middle part, that is, inside the package, or that is even unsuitable as a material inside the package. Another problem may also be posed by insufficient mechanical properties, for example insufficient rigidity, of the edge flange. The choice of materials for the inner part of the package should not excessively determine the materials for the edge flange. A method is also known to form the edge flange independently of the rest of the package, as disclosed in international patent document WO 03/078012 and the corresponding US application publication 2004/0262322, presenting a method in which the rim of a tray-like package is moulded of a plastic material separately. This document discloses a solution, in which the blank is provided with the joined together at their edges with moulded plastic material, after which the plastic rim is moulded to the upper edges of said side walls, to form a flange extending outwardly from them and encircling the package. Publications JP 8207924 and JP 10034702 describe also this kind of solutions wherein the edge rim of a tray-like package is injection moulded separately. In these publications the tray is composed from separate side-wall and bottom parts being joined to each other by forming before the injection moulding phase of the edge rim.
According to publication 2004/0262322, the packages made by press-forming are always first equipped with the edge flange, to which the plastic material is attached by moulding, for example under the flange or at its edge, as an extension of the flange outwards, as shown, for example, in FIGS. 59 to 63 and 66 of the publication. Furthermore, the document mentions the possibility of forming a tray-like package “in-mold”; in other words, the press-forming and the moulding of the plastic material to the edge take place in the same mould. This is shown in more detail in FIGS. 70 to 76 of the publication, which illustrate the feeding of moulding material underneath the edge flange all the way to the outer surface of the side wall and as a horizontal extension to the upper surface of the flange, and in FIGS. 77 to 80, which illustrate a mould for implementing this. The upper surface of the edge flange of the package, that is, the surface coming against the lid, is tightly against a shutoff surface during the moulding phase. The mould may comprise several openings for supplying moulding material at the circumference of the package. Also the publication JP 60058823 describes a method to form a container provided with edge rim being injection moulded in a same mould as where a tray of the container (having D-shaped side wall) has been formed. In this solution the side wall and bottom of the tray has been formed from one blank comprising side wall part and bottom part being shaped and connected to each other during the moulding phase with the help of moulding material. Furthermore, US patent application publication 2007/267374 discloses also method for forming this type of packages. In this document the tray is formed by means of press forming. After the press forming the edge flange of the container is injection moulded to the upper edge of the tray in same mould wherein the press forming has been performed. The document describes several embodiments wherein different kinds of edge flanges are shown. In these embodiments the edge flange or rim is injection moulded either on to the outer surface or on to the inner surface or such that it encapsulates fully the upper edge of the tray. This document represents the closest prior art in view of the present invention. The dimensions of the forming and moulding tool, especially the shapes of the surfaces defining the mould cavity, determine the final dimensions of the plastic part at the edge of the package, which dimensions must be taken into account already when designing the forming tool. It must be possible to keep the cardboard blank with a given holding force between the shutoff surfaces of the mould up to the end of the forming step, so that the package is formed evenly at all sides and a good-quality product is obtained. By the method presented in the publication, the plastic part can only be moulded underneath the edge flange.
The patent document U.S. Pat. No. 4,960,376 discloses a mould apparatus for forming a container wherein mould is formed by means of side, end and corner sections being movable in respect of each other. The annular cavity encircling the main cavity for forming the body of the container functioning as a mould for the handling ring (i.e edge flange) is formed to the moulding apparatus by moving the corner section backwards from the surface opposing to it before injecting the moulding material in to this cavity. After extruding phase the shape of this cavity is changed by moving the end section and the corner section of the apparatus such that a desired shape of the edge flange is formed and simultaneously the mould material is compressed against the corner of the body of the container such that these are welded to each other. Although this kind of mould apparatus improves the possibilities of changing the shapes of the moulding cavity it does not give any solution to the above mentioned problem with the holding surface of the upper edge of the tray-like container. It has also other drawbacks since it requires separate injection (extruding) and compression phases while the sections of the mould apparatus has to be moved back and forth making the implementation and use of the method much more complicate as in the methods described in above mentioned other documents.